The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefins. There are numerous technologies available for producing oxygenates including fermentation or reaction of synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials including coal, recycled plastics, municipal waste or any other organic material.
The preferred methanol conversion process is generally referred to as a methanol-to-olefins (MTO) process, where methanol is converted primarily to ethylene and/or propylene in the presence of a molecular sieve which in turn can be used as the basic ingredients for polymers such as polyethylene and polypropylene. Molecular sieves have a crystalline pore structure with uniform sized pores of molecular dimensions that selectively adsorb molecules that can enter the pores, and exclude those molecules that are too large.
There are many different types of molecular sieves to convert a feedstock, especially a feedstock containing an oxygenate, into one or more olefins. For example, in U.S. Pat. No. 4,310,440 is disclosed a process of producing light olefin(s) from an alcohol using crystalline aluminophosphates, often represented by ALPO4. The most useful molecular sieves for converting methanol to olefin(s) are silicoaluminophosphate molecular sieves.
These molecular sieves have been found to be sensitive to various contaminants resulting in the lowering of the yield of light olefins and even affecting the operability of a conversion process. Such contaminants are introduced to a particular conversion process in a variety of ways. Sometimes the molecular sieve itself produces contaminants affecting the conversion performance of the molecular sieve. In addition, in large scale processes, it is more likely that the effect of various contaminants entering into commercial conversion processes is higher. Contaminants can be introduced into the oxygenate feedstock or in the air that is introduced, especially into the catalyst regeneration unit. Unfortunately, it has been found that contaminants such as salts become concentrated over time to the extent that olefin yields are significantly impacted. In addition, the exposure of the catalyst to very high temperature steam in the regeneration unit has a significant contribution to the deactivation of the catalyst. We refer to this deactivation as “hydrothermal deactivation.” Temperatures in the regeneration unit are typically about 625° C. or higher as compared to about 475° C. in a methanol-to-olefins reactor. Due to the adverse effects of these higher temperatures upon catalyst activity, in the present invention it has been found very important to keep the moisture level as low as reasonably possible within the regeneration unit.
Therefore, it would be highly desirable to control contamination so as not to adversely affect the molecular sieve catalyst. Controlling contamination is particularly desirable in oxygenate to olefin reactions, particularly in methanol to olefin reactions, where feedstocks and catalysts are relatively expensive. It has now been found highly desirable to dry the air to the regeneration unit in order to significantly reduce the rate of catalyst deactivation caused by exposure to steam in the regeneration unit.
In addition, it has been previously reported by Janssen et al. in US 2004/0034264 A1 and US 2004/0034265 A1 that feedstocks need to be free or substantially free of salts. However, it has now been found that serious damage to the catalyst can be caused by exposure of the catalyst to the sodium chloride that is present in the air in coastal areas such as where petrochemical plants are frequently located. The present invention provides a process to protect the catalyst from harm from this and other salts that may be unexpectedly present in the air entering the reactor and particularly regarding air entering the catalyst regeneration vessel.